Then WP moves on to the test which Bryan first talked about in a comment to my post and I posited was probably the better test - the use of commas as markers for descriptive phrases (not present in restrictive phrases). He agrees that this is a better test than the judge's offered that/which test. He quoted Strunk and White (and I may have to go out and buy the book because the example is dead on for my purpose), however, since I've yet to travel to my local brick and mortar, I'll quote from my 1896 Higher Lessons in English:

The adjective clause, when not restrictive, is set off by a comma.

In plain language I think the difference is this:

The puppy, which is a beagle, is cute.

The puppy which is a beagle is cute.

In the first sentence you are describing the only puppy in the room to someone. In the second sentence you are at a pet store staring through a window at 20 puppies and are describing the specific puppy you are talking about. The first doesn't modify the meaning of the sentence; whether you include ", which is a beagle," or not you are talking about the same dog. The second does modify the meaning; "which is a beagle" defines which - of 20 puppies - you are talking about.

With that, I'm off to my local brick & mortar to purchase an English text published more recently than 1894.

The second construction is discouraged by some of the handbooks I found, but none set an absolute rule against it.

Thus, when writing, it is arguably better to use which only for non-restrictive clauses because that's what many people expect. But when reading, we can't rely on which to identify non-restrictive clauses since not everyone follows that rule.

For example, the King James version of the bible is full of restrictive whichs: "Let us now go even unto Bethlehem, and see this thing which is come to pass[...]" (Luke 2:15 KJV)

And here's one from Moby Dick: "Between the marble cenotaphs on either hand of the pulpit, the wall which formed its back was adorned with a large painting [...]"

The Adventures of Huckleberry Finn is full of them, but that's Huck talking, and he's not exactly an authority.

We're better off assuming that non-restrictive clauses will be set off by commas, regardless of the relative pronoun used.

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Disclaimer

In case anyone out there needs this warning: This ain't legal advice. Everything in the blog is off the cuff and no one goes back and reads all the cases and statutes before blogging. The law may have changed; cases misread and misunderstood two years ago can still lead to a clinging misperception. Courts in your county, city, or State probably don't operate as described herein. Feel free to be inspired, but YOU MUST ALWAYS DO YOUR OWN RESEARCH OR HIRE A COMPETENT ATTORNEY TO DO SO because I haven't.